1. Field of the Invention
This invention relates to a method and apparatus for recording and reading out a radiation image. This invention particularly relates to a method and apparatus for recording and reading out a radiation image, with which a visible radiation image of an object having good image quality with enhanced sharpness and enhanced graininess is obtained.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal, and the electric signal (image signal) is processed and then used for reproducing the X-ray image as a visible image on a photocopy, or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like, can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays, such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, 4,543,479, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used during the reproduction of the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT) display device, or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials, in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light which the stimulable phosphor sheet emits when being stimulated varies over a wide range and is proportional to the amount of energy stored thereon during its exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order for the desired image density to be obtained, an appropriate read-out gain is set when the emitted light is being detected and converted into an electric signal (image signal) to be used in the reproduction of a visible image on a recording material, such as photographic film, or on a display device, such as a CRT display device.
In the radiation image recording and reproducing systems described above, when a radiation image of an object is recorded on a stimulable phosphor sheet, the stimulable phosphor sheet is ordinarily located such that it may extend in a direction which is normal to the direction of travel of the radiation carrying image information of the object.
It is desired that the radiation images obtained with the aforesaid radiation image recording and reproducing systems, or the like, have good image quality and can serve as effective tools in, particularly, the efficient and accurate diagnoses of illnesses.
Such that a radiation image having good image quality may be obtained, it is considered to increase the thickness of the stimulable phosphor layer of the stimulable phosphor sheet.
Specifically, if the thickness of the stimulable phosphor layer of the stimulable phosphor sheet is increased, a larger amount of radiation can be absorbed by the stimulable phosphor layer, and a larger amount of energy can thereby be stored on the stimulable phosphor layer during the image recording operation. As a larger amount of energy is stored on the stimulable phosphor layer, a larger amount of light can be emitted by the stimulable phosphor layer when it is exposed to stimulating rays during the next image read-out operation. As a result, a visible image having higher contrast and higher graininess can be reproduced from the image signal obtained by guiding and detecting the emitted light.
However, when the stimulating rays are irradiated to the stimulable phosphor layer having an increased thickness, the stimulating rays are diffused in accordance with the thickness of the stimulable phosphor layer. Therefore, the sharpness of the visible image reproduced from the image signal, which is obtained by guiding and detecting the light thus emitted by the surface of the stimulable phosphor layer, becomes low. Accordingly, an image having good image quality cannot be obtained.